Register Object inside of Another Object
Ahhhh! It seems I had to remove Default value from CollectionProperty. It was my mistake. Here is working code:

class MR_CurvePoint(bpy.types.PropertyGroup):
    y = BoolProperty(
        name="y",
        description="y...",
        default=False
    )


class MR_CurveObject(bpy.types.PropertyGroup):
    x = CollectionProperty(
        name="x",
        description="x...",
        type = MR_CurvePoint,
    )

...

def register():
    bpy.types.Scene.my_curve_object = CollectionProperty(
        name="Mira Tool Variables",
        type=MR_CurveObject,
        description="Mira Curve"
    )